The present invention relates to the field of microelectromechanical (MEM) systems, and in particular to a compact electrostatic actuator that can be formed by surface micromachining.
Many different types of microelectromechanical (MEM) devices use one or more electrostatic actuators as motive sources. See, for example, U.S. Pat. No. 5,631,514 to Garcia et al which discloses a microengine that uses a pair of synchronized linear comb actuators as a power source. However, many problems exist with present electrostatic comb actuators which are generally considered to be low power, low output force devices.
Conventional comb actuators are disproportionate in size compared to other functional elements (e.g. gears, linkages, moveable assemblies, etc.) of MEM devices due to inefficient space utilization. Since a total substrate area (i.e. a die size) devoted to a particular MEM device is generally fixed by fabrication constraints, the disproportionate area required for one or more comb actuators limits the area available for use by the other functional elements of the MEM device. As a result, the complexity of a MEM device utilizing a conventional electrostatic comb actuator will ultimately be limited by the area required for each actuator.
Another problem with conventional electrostatic comb actuators is that these devices provide a relatively low drive force limited to a few tens of microNewtons (xcexcN); and this level of drive force requires a relatively high operating voltage of up to 100 Volts or more. The relatively low drive force of conventional comb actuators limits their usefulness as power sources for many types of MEM devices.
Yet another problem with conventional electrostatic comb actuators is that a single-beam structure is commonly used; and this single-beam structure and one or more electrostatic combs supported thereon can easily be distorted due to slight asymmetries in a generated electrostatic field, or due to external loading. Such distortion can result in one or more fingers of an electrostatic comb bending or moving from a region of electrostatic stability to a region of electrostatic instability. This can result in binding of the comb actuator and limiting its range of motion. Permanent failure of the actuator can also occur as an electrical short circuit is developed between contacting fingers which are biased at different electrical potentials.
The compact electrostatic actuators of the present invention overcome many of the above limitations in the prior art.
An advantage of the present invention is that a compact design is provided for an electrostatic actuator to conserve space upon a substrate or portion thereof, thereby providing for more economical fabrication and allowing the fabrication of MEM devices of increased complexity. Embodiments of the electrostatic actuator of the present invention are provided as electrostatic comb actuators and as capacitively-coupled electrostatic plate actuators.
Another advantage of the electrostatic actuator of the present invention is that this actuator can provide an increased drive force as compared with a conventional comb actuator.
A further advantage of the present invention is that a comb actuator is provided with a more rigid structure that is less prone to distortion than a conventional electrostatic comb actuator, thereby increasing the reliability of the comb actuator even at high actuation voltages.
Still another advantage of the present invention is that an electrostatic actuator can be formed which provides a self-limiting displacement, thereby making the actuator relatively insensitive to the exact value of the applied voltage and acting to resist any further displacement due to an increase or decrease in loading.
Yet another advantage is that an electrostatic actuator according to the present invention can be provided with electrostatic shielding in critical places to minimize the effects of unwanted electrostatic fields, thereby increasing the available electrostatic drive power from the actuator and reducing any distortion of the structure of the actuator.
These and other advantages of the present invention will become evident to those skilled in the art.
The present invention relates to an electrostatic actuator formed on a substrate and comprising a plurality of capacitively-coupled electrostatic plates (also termed electrodes) including at least one row of fingerless stationary electrostatic plates formed on the substrate and oriented along a direction of movement of the electrostatic actuator, and a row of moveable electrostatic plates formed on at least one side of each row of the stationary electrostatic plates, with the moveable electrostatic plates being suspended above the substrate by a rigid frame which supports a majority of the moveable electrostatic plates at or near a midpoint thereof, the frame further surrounding and enclosing each row of the stationary electrostatic plates. In the electrostatic actuator, the frame is suspended above the substrate by a plurality of springs, with each spring being attached at one end thereof to the frame and attached at the other end thereof to the substrate. This allows the frame to be electrostatically moveable over a range of generally 0-10 microns in response to a voltage applied between the stationary electrostatic plates and the moveable electrostatic plates.
The frame can be formed from a plurality of layers (e.g. xe2x89xa73 layers) of a deposited and patterned electrically-conductive material (e.g. polycrystalline silicon when the substrate comprises silicon). Additionally, the frame comprises a plurality of links for supporting the majority of the moveable electrostatic plates at or near the midpoint thereof, with each link generally being formed from a smaller number of deposited and patterned layers than are used to form the remainder of the frame. A ground plane can be formed on the substrate below the frame at the location of each row of the moveable electrostatic plates; and an electrostatic shield can be provided between the frame and an adjacent stationary electrostatic plate.
The spacing between each moveable electrostatic plate and an adjacent stationary electrostatic plate can be made substantially the same for all of the stationary and moveable electrostatic plates. This allows the electrostatic actuator to be driven by a voltage that is simultaneously applied between all the stationary electrostatic plates and all the moveable electrostatic plates. Alternately, the stationary electrodes can be digitally addressed to provide the voltage between at least one set of the stationary electrostatic plates and the moveable electrostatic plates. In some preferred embodiments of the present invention, the spacing between adjacent moveable electrostatic plates in each row of the moveable electrostatic plates can be made substantially equal to the spacing between adjacent stationary electrostatic plates in each row of the stationary electrostatic plates.
The present invention further relates to an electrostatic actuator formed on a substrate and comprising a plurality of moveable electrodes suspended above the substrate and arranged in at least one row oriented along a direction of movement of the electrostatic actuator; a rigid frame surrounding and supporting the moveable electrodes; and a plurality of fingerless stationary electrodes formed on the substrate in a row proximate to the row of moveable electrodes, the rows of stationary and moveable electrodes acting in response to a voltage applied therebetween to urge the frame to move along the direction of movement.
The frame is suspended above the substrate by a plurality of springs, with each spring being attached at one end thereof to the frame and attached at the other end thereof to the substrate. All the electrodes of a given type (i.e. stationary or moveable) can be electrically connected together when forming a unidirectional device so that the actuator can be operated with a common voltage provided between the stationary electrodes and the moveable electrodes. Alternately, the stationary electrodes can be digitally addressed to provide the voltage between at least one set of the stationary electrostatic plates and the moveable electrostatic plates. With the spacing between each moveable electrode and a nearest stationary electrode being substantially equal for all of the stationary and moveable electrodes, an additive electrostatic force is generated to move the frame and thereby provide a displacement to a load.
The frame comprises a plurality of layers of a deposited and patterned electrically-conductive material (e.g. polycrystalline silicon when the substrate comprises silicon). The frame further includes a plurality of links for supporting a majority of the moveable electrodes at or near a midpoint thereof, with each link generally being formed from a smaller number of layers of the electrically-conductive material than are used to form the remainder of the frame to provide a large change in capacitance with displacement. A ground plane is generally formed on the substrate below each row of the moveable electrodes which is also generally maintained at ground electrical potential. An optional electrostatic shield can be provided on the substrate between the frame and one or more adjacent stationary electrodes.
The present invention also relates to an electrostatic actuator formed on a substrate and comprising a plurality of stationary electrodes formed on the substrate and regularly spaced in at least one row oriented along a direction of movement of the electrostatic actuator; a rigid frame supported above the substrate by a plurality of springs, with the frame surrounding each row of the stationary electrodes; and a plurality of moveable electrodes located on the frame on both sides of each row of the stationary electrodes, the moveable electrodes acting in combination with the stationary electrodes, which are either all connected together or configured as sets of electrodes, to displace the frame in the direction of movement in response to an applied voltage.
The moveable electrodes are supported on the frame in the electrostatic actuator by a plurality of links connecting the moveable electrodes and forming a plurality of rows of the moveable electrodes. Each link preferably has a thickness that is smaller than the thickness of the moveable electrodes (e.g. less than one-half the thickness of the moveable electrodes) to provide a large change in capacitance with displacement. The frame can comprise electrically-conductive polycrystalline silicon when the substrate comprises silicon. A ground plane is preferably formed on the substrate below the frame underneath the moveable electrodes; and an optional electrostatic shield can be formed on the substrate between the frame and an adjacent stationary electrode.
Additionally, the present invention relates to an electrostatic actuator which comprises a substrate (e.g. comprising silicon); a plurality of fingerless stationary electrodes formed on the substrate and arranged in at least one row oriented along a direction of movement of the electrostatic actuator; a plurality of moveable electrodes suspended above the substrate, with the number of moveable electrodes being at least equal to the number of stationary electrodes, and with at least one of the moveable electrodes acting in combination with each stationary electrode to generate an electrostatic force in response to a voltage applied therebetween to urge the moveable electrodes to move in the direction of movement; and a rigid frame surrounding and supporting the moveable electrodes above the substrate, with the frame being attached to the substrate by a plurality of springs.
The frame, the stationary electrodes and the moveable electrodes comprise a plurality of layers of a deposited and patterned electrically-conductive material (e.g. polycrystalline silicon). The frame further comprises a plurality of links for supporting a majority of the moveable electrodes, with each link being formed from at least one layer of the electrically-conductive material. A ground plane can be formed on the substrate below the moveable electrodes; and an electrostatic shield can be formed on the substrate between the frame and an adjacent stationary electrode.
The present invention further relates to an electrostatic actuator formed on a substrate (e.g. silicon) and comprising a plurality of fingerless stationary electrodes formed on the substrate from a plurality of layers (e.g. xe2x89xa73 layers) of deposited and patterned electrically-conductive material (e.g. polycrystalline silicon); a plurality of moveable electrodes suspended above the substrate proximate to the stationary electrodes, with the moveable electrodes being formed from the plurality of layers of deposited and patterned electrically-conductive material; a plurality of links connecting the moveable electrodes, with the links being formed from the same number of layers or preferably fewer layers (e.g. a single layer) of the deposited and patterned electrically-conductive material than used to form the moveable electrodes; and a rigid frame surrounding and supporting the links and moveable electrodes, with the frame being formed from the plurality of layers of deposited and patterned electrically-conductive material and further being moveable in response to a voltage applied between the stationary and moveable electrodes.
The present invention also relates to an electrostatic actuator formed on a silicon substrate and comprising a plurality of stationary electrodes formed on the substrate from at least three layers of deposited and patterned polycrystalline silicon; a plurality of moveable electrodes suspended above the substrate proximate to the stationary electrodes, with the moveable electrodes being formed from at least three layers of deposited and patterned polycrystalline silicon; a plurality of links formed from at least one layer of deposited and patterned polycrystalline silicon, with each link connecting a pair of the moveable electrodes; and a frame formed from at least two layers of deposited and patterned polycrystalline silicon for supporting the links and moveable electrodes and further being moveable in response to a voltage applied between the stationary and moveable electrodes.